1. Field of the Invention
The present invention is related to a method of inspecting for overly shift defects during semiconductor manufacture and apparatus thereof, and more particularly, is related to a method of inspecting for overlay shift between contact/via plugs and metal lines in a semiconductor device using charged particle beam imaging.
2. Description of the Prior Art
Overlay shift is a common defect which occurs during the manufacture of semiconductor devices (such as integrated circuits). One of the major causes of the overlay shift defect stems from misalignment of photolithography masks, which are used for forming two successive layers within a semiconductor device. FIG. 1 is a cross-sectional view showing an overlay shift defect in a semiconductor device. As shown in FIG. 1, overlay shift defects can happen in various layers of a semiconductor device, for example, at the location between a contact plug 104 and a first layer metal line 102, or the location between a via plug 108 and a second layer metal line 106. Because of overlay shifting, the formed metal lines can fail to be aligned with the underlying contact/via plugs after etching. This can result in a high contact resistance between the elements. Such resistance can cause malfunction of the entire device; for example, a disconnected source/drain contact plug with the corresponding bit line can obstruct the operation of a DRAM device.
Another issue brought about by this type of defect is the manufacturing cost. As a semiconductor device is generally fabricated in a layer-by-layer fashion with one layer on top of another, a defective plug-to-contact layer should be reported immediately to enable stoppage of the process so that further waste can be avoided.
Currently, the inspection for defects on a semiconductor sample is mostly carried out through charged particle beam imaging of the sample. FIG. 2 is a schematic illustration of a typical charged particle beam microscope system 200 according to the conventional art. A charged particle beam source 210 generates a charged particle beam, and then the charged particle beam is condensed and focused by a condenser lens module 220 and an objective lens module 230, respectively, to form a charged particle beam probe 240. The formed charged particle beam probe 240 then bombards the surface of a sample 295 secured on a stage 290. Charged particle beam probe 240 is controlled by a deflection module 250 to scan the surface of sample 295. After charged particle beam probe 240 bombards the surface of sample 295, secondary charged particles 260 are induced to emit from the sample surface along with other charged particles of beam probe 240 reflected by sample 295. These particles are then detected and collected by a detector module 270. Then, detector module 270 generates a detection signal 271 accordingly. An image forming module 280 coupled to detector module 270 then receives detection signal 271 and accordingly forms a charged particle microscopic image of sample 295. In one example, the charged particles are electrons.
The presence of an overlay shift defect, however, can be difficult to detect using existing charged particle beam inspection methods and tools. Currently, monitoring for this defect is typically implemented through human observation and interpretation of a top-down view charged particle microscopic image of the sample, with the result more or less arbitrary and the accuracy poor.
Therefore, it is desirable to have a method for reliably and timely (e.g., in-time) reporting the presence of overlay shift defects after formation of a metal line on top of a contact/via plug (typically by an etching process) in a semiconductor device for the purpose of manufacture process control.